EP0243885B1 - Verfahren und Vorrichtung zum Verbindungsaufbau bei Kurzwellenfunknetzen - Google Patents

Verfahren und Vorrichtung zum Verbindungsaufbau bei Kurzwellenfunknetzen Download PDF

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Publication number
EP0243885B1
EP0243885B1 EP87105972A EP87105972A EP0243885B1 EP 0243885 B1 EP0243885 B1 EP 0243885B1 EP 87105972 A EP87105972 A EP 87105972A EP 87105972 A EP87105972 A EP 87105972A EP 0243885 B1 EP0243885 B1 EP 0243885B1
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EP
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Prior art keywords
signals
signal
present
frequency
decision
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EP87105972A
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German (de)
English (en)
French (fr)
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EP0243885A3 (en
EP0243885A2 (de
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Roland Küng
Hanspeter Widmer
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Ascom Radiocom AG
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Ascom Radiocom AG
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Priority to AT87105972T priority Critical patent/ATE99101T1/de
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Publication of EP0243885A3 publication Critical patent/EP0243885A3/de
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/24—Radio transmission systems, i.e. using radiation field for communication between two or more posts

Definitions

  • the invention relates to a method and a receiving device for synchronizing receivers of a shortwave radio network with one transmitter each of this network for the purpose of establishing a connection in accordance with the preambles of the independent claims.
  • Shortwave connections mainly use the propagation of spatial waves, which are reflected on the ionosphere, in order to realize a message transmission over large distances.
  • a spatial wave connection such as noise-like channel interference, time-varying, dispersive channel behavior and the presence of selective interferers - this type of transmission has recently gained in importance, thanks to new microprocessor technologies and low costs compared to satellites.
  • Today's usual transmissions are carried out for the economical use of the frequency supply by means of single-sideband technology, with the frequency of the signal from the audio frequency band (300 Hz to 3.4 kHz) being carried out on the transmitter side into a selected HF band and the reverse operation being carried out by the HF receiver.
  • the received signal is passed on to demodulator and decoder circuits in the LF range.
  • the HF receivers have automatic gain controls, with the total power or voltage forming the controlled variable within the selected receive channel bandwidth.
  • different noise and useful levels are set at the output, within wide limits. In particular, selective interferers, which have more signal energy than the useful signal, are frequently encountered and the channel is then usually considered to be occupied.
  • the selective call transmitters and receivers of the individual stations are accommodated in their modulator or demodulator block.
  • the call signals are composed of a set of suitable amplitude-time functions which can be recognized by the individual receivers in the channel noise and distinguished from one another. Even if the transmission quality is poor, on the one hand no wrong stations and on the other hand the desired stations should always be activated.
  • an HF radio transmission method is known from the document WO 82/02633, in which the signals are encoded in succession with five double-tone channels.
  • a respective synchronization signal ensures precise timing in the receivers.
  • This synchronization signal also consists of five double-tone mark and space signals, which alternate serially at a rate of 10 Hz, the frequency of these signals being in the range from 400 Hz to 2.7 kHz.
  • This object is achieved according to the invention by using a distinctive synchronization signal which is adapted to the transmission channel and which consists of narrowband mark and space signals which form the partial signals of a diversity pair.
  • the use of the synchronization signal according to the invention has the advantage that, at the same time as the frequency offset is determined, bit synchronization between the stations is made possible by determining the phase of the modulation signal modulating the carrier signal at the receiving location.
  • the modulation signal is obtained mathematically precisely since the expected signal is known.
  • an increase in the probability of incorrect synchronization caused by the presence of certain disturbances can be largely avoided.
  • the structure of the synchronization signal from narrow-band mark and space signals, which form the partial signals of a diversity pair opens up the possibility of separate detection of these partial signals, which considerably increases the reliability of the connection setup elevated. This is because the probability that there is an interferer striking the marking signal at the same time in both diversity channels is zero.
  • a frequency offset of a few Hz between the interferer and the marker signal is not critical, since 500 subchannels of 1 Hz each are examined in the range between 250 and 750 Hz using a special signal processing operation.
  • the invention further relates to a device for carrying out the method mentioned, with a synchronization signal receiver.
  • the device according to the invention is characterized in that the synchronization signal receiver has means for the independent detection and evaluation of the two partial signals of the diversity pair as well as means for comparing the results obtained therefrom.
  • a short-wave connection that is commonly used today consists of a transmitter 1 and a receiver 2 between which the signals are transmitted via a transmission medium 3.
  • the transmitter-side data input takes place in a modulator / code circuit 4, to which a time base 5 is assigned.
  • the output signal of the modulator / encoder circuit 4 is an LF signal in the audio frequency band between 300 Hz and 3.4 kHz.
  • the transmitter 1 which is an RF SSB transmitter, performs a frequency translation into a selected RF band.
  • a frequency base 6 in the range of the HF band is assigned to the transmitter 1.
  • the RF output signal of the transmitter 1 emitted in the time-variant transmission medium 3 is, for example, in the range between 3 and 30 MHz.
  • Additive noise ST is added to this HF signal in transmission medium 3.
  • the received RF signal is transferred to an LF signal in the transmitter's audio frequency band and fed to a demodulator / decoder circuit 7, which is assigned a time base 5'.
  • the data is output at the output of the demodulator / decoder circuit 7.
  • a shortwave radio network forms a so-called selective call network
  • each of the stations involved has a selective call transmitter and receiver, which are accommodated in the modulator and demodulator blocks 4 and 7 of the arrangement in FIG. 1 (see, for example, DE-PS 32 11 325).
  • the signals for calling, the so-called call signals are composed of a set of suitable amplitude-time functions that can be recognized by the individual receivers in the channel noise and distinguished from one another.
  • FIG. 2 schematically shows a call signal used in the method according to the invention. As shown, this consists of a synchronization signal SS and an address signal AS.
  • the receiver looks at time intervals of length T and decides whether or not a synchronization signal SS is present within the respective interval.
  • the observation intervals are weighted using a window function (FIG. 4).
  • a time period of 2 s is preferably reserved for the synchronization signal SS. So that at least one observation interval completely overlaps with the transmitter signal in the initial, desynchronous state, T may be at most s.
  • the length T of the observation interval is only sensibly selected if it is shorter than the coherence time T c of the received signal. With the selected time window, T c > T should be.
  • the receiver does not know the exact carrier frequency of the transmitter, but there is an expectation range in which a call signal occurs with the highest probability. Depending on the technology of the transmitter and receiver, this range of expectations can include up to 500 Hz and is ⁇ 234 Hz in the exemplary embodiment described. Within this A call signal should be perfectly detectable in the range and its frequency offset should be determined to an accuracy of at least ⁇ 1 Hz depending on the signal / noise ratio. Clear detection should be possible for a signal / noise ratio of up to at least -24 dB, based on 2 kHz bandwidth.
  • synchronization signal SS which is adapted to the transmission channel and can be easily detected in a disturbed environment.
  • This synchronization signal SS emitted during the period T o is a low-frequency carrier signal which frequency-modulates with a square-wave function and is also known as an FSK signal. As shown, it consists of "mark" and "space” signals.
  • the synchronization signal SS enables bit synchronization between the stations simultaneously with the determination of the frequency offset, in that the phase of the modulation signal is determined at the receiving location.
  • the modulation frequency is quartz-precise and is known to the receiver.
  • the phase should be able to be determined with an accuracy of at least 0.5 rad.
  • the mark and space signals each an AM signal in themselves, are narrow-banded to ensure a uniform variation of the most intense spectral components with selective fading.
  • the frequency spacing between them is chosen to be as large as possible in order to obtain two signals decorrelated with respect to selective fading, but both of which lie within the same channel.
  • the keying frequency is significantly higher than the fading frequency and differences in transit time should play a minor role.
  • a modulation frequency of 16 Hz is selected, a baseband carrier around 2 kHz for the mark signal and a baseband carrier around 500 Hz for the space signal.
  • both carriers can be varied in order to allow adaptive shifts of the AM signals do.
  • Mark and space signals are considered by the receiver as an AM diversity pair and are detected separately. This has the additional advantage that the detection reliability increases significantly if the interference signal distribution on the channel is uneven.
  • the overall signal has constant power (no FSK, AM component), enables non-linear amplifier operation and optimal use of the transmitter stage and is also clearly distinguishable from selective interference signals.
  • the HF receiver If the HF receiver is in automatic scan mode, for example SELSCAN (registered trademark of Rockwell-Collins), it periodically examines a certain number of programmed channels for a possible synchronization signal. This is sent out by the transmitter as long as a scan cycle lasts. After successful detection of a synchronization signal, the receiver stops the scanning operation and waits for the address signal AS (FIG. 2).
  • SELSCAN registered trademark of Rockwell-Collins
  • the receiver looks at time intervals of length T and decides whether a synchronization signal is present within the respective intervals.
  • the observation intervals are weighted with a window function.
  • a synchronization signal SS of length T o is shown in line a, the windows of the observation intervals in lines b and c (not true to scale), and in line b the even-numbered windows F n - 2 , F n , F n +2 etc. and in line c the odd F n - 1 , F n + 1 ' etc.
  • the length T of an observation interval is 1 s and is determined by the length T o of the synchronization signal SS and by the coherence time T c of the channel.
  • Detection values of two overlapping observation intervals are practically statistically independent due to the window function, so that there is approximately a time T during the transmission of the synchronization signal SS Have the detection values removed.
  • the suitable selection of the window function enables high dynamics in the spectral range after the fast Fourier transformation FFT has been carried out (FIG. 7a).
  • the receiver therefore continuously accumulates detection values in a "lossy integrator" or in a digital low-pass filter.
  • the desired components crystallize from the stochastic components bit by bit, similar to a puzzle, so that up to a certain usable integration time, an increasingly sharper image of the synchronization signal emerges, from which both the carrier frequency and the phase angle are determined can.
  • the minimum signal / noise ratio required for successful detection and synchronization can thus be reduced within certain limits, depending on the duration of the transmission of the synchronization signal, to approximately -24 dB with a 2 kHz noise bandwidth.
  • synchronization signal SS After the transmission of the synchronization signal SS and its detection, all selective call receivers on the same call channel are synchronized.
  • the synchronization signal SS is immediately followed by an address signal AS which makes the actual selective call.
  • word synchronization that is to say the complete time synchronization between transmitter and receiver, is then also established.
  • the receiver carries out two independent detections and evaluations of the two partial signals of the diversity pair and subsequently compares the results.
  • the two additively disturbed received signals are converted into a sequence of N numerical values by an A / D converter after prior analog processing (filtering and mixing) during each observation period T.
  • the term receiver refers to a demodulator / decoder in the LF frequency range (cf. demodulator / decoder 7 in FIG. 1).
  • the received signal r (t) is first passed through an overall channel filter 8 with a pass band from 300 Hz to 3.4 kHz, to the output of which two paths 9 A and 9 B are connected for the two partial signals of the diversity pair.
  • a first mixer 10 A or 10 B With a first mixer 10 A or 10 B , the signals in each path are mixed up with a variable oscillator into the same reception band A or B (see FIG. 6) and then filtered with an IF filter 11 A , 11 B , whose Pass curve is around 4.5 kHz.
  • An AGC amplifier 12 is connected to each of the IF filters 11 A , 11.
  • the two frequency ranges Mark and Space of 500 Hz bandwidth are mixed down in each path 9 A , 9 B with a second mixer 13 into the baseband from 250 Hz to 750 Hz, which is used as a fixed processing band.
  • This is followed by filtering with an image frequency filter 14 A , 14 B for the purpose of attenuation.
  • the output signals r A (t) and r B (t) of the image frequency filters 14 A and 14 B each arrive in a sampler 15 with a downstream A / D converter 16, at the output of which there is a signal vector rA or r.
  • the signal vectors r and r each contain N values which initially go into a buffer memory 17, from where they can be called up by a signal processor.
  • the buffer store 17 consists of three partial stores of the size ; one part is available to the A / D converter 16 and two parts to the processor for processing.
  • the dashed curve H 8 (f) corresponds to the transmission characteristic of the overall channel filter 8
  • the dash-dash curve H 14 (f) to that of the image filter 14 A , 14 B
  • the arrow P represents the scanning signal.
  • the sampling frequency is shown as 2.048 kHz.
  • the characteristic curve H c (f) represents the fixed processing band (baseband from 250 to 750 Hz)
  • the characteristic curve H A (f) the variable reception band for the one partial signal (path 9 A , FIG. 5)
  • the characteristic curve H B (f ) the variable reception band for the other partial signal (path 9 B , Fig. 5) of the diversity pair.
  • H 11 (f) is the transmission curve of the IF filter 11 A , 11 B - (FIG. 5).
  • FIG. 7 shows the individual functional levels of the signal processing as it is carried out by the corresponding part of the synchronization receiver formed by a signal processor.
  • signal vector r A only half of the diversity receiver is considered (signal vector r A ), since this is constructed completely symmetrically.
  • r B With the second signal vector (r B ), the same signal processing takes place in the processor as with the first (r A ), only with different numerical values.
  • Fig. 7 is for the sake of Clearly divided into two figures, 7a and 7b.
  • Fig. 7a shows the signal processing up to the so-called hypothesis decision and Fig. 7b shows the remaining functional levels.
  • the result of the signal processor after the numerical signal processing contains the selected hypothesis whether a synchronization signal is present (H 1 ) or not (H o ).
  • H 1 an estimate is made for the frequency offset and the phase of the two signals r A and r B and for the values of their signal / noise ratio.
  • the numerical signal processing which is carried out in real time, essentially checks whether the reception vector r of the N-dimensional vector space IR lies in the decision area of the hypothesis H 1 or Ho.
  • the decision area has the shape of an N-dimensional cone with the tip in the origin of the space IR.
  • the amount of r does not affect this decision.
  • the hypothesis value is based solely on the direction of r .
  • the decision area is therefore an N-dimensional solid angle area.
  • the investigation of r with regard to its decision area is carried out using the computing algorithms described below in connection with FIG. 7, which represent linear and nonlinear coordinate transformations.
  • the first arithmetic operation that takes the N values of the signal vector r A (and also r B , which, as mentioned, is not shown) are subjected to the weighting with the window function F, followed by a Fourier transformation.
  • the latter maps the vector r of the space IR into the space IR '.
  • the Fourier transform used is a so-called fast Fourier transform FFT, which represents the computationally faster version of the discrete Fourier transform. Since the synchronization signal is periodic in nature, the transition to the frequency range results in a separation into actual signal and noise components. This separation in the manner of a filtering is better, the higher the spectral resolution of the Fourier transformation. The resolution in turn is determined by the observation time T or the "size" of the FFT.
  • the part of the signal processing that follows the Fourier transformation FFT is used for demodulating (identifying) the diversity pair, for noise estimation, for signal integration (accumulation) for hard-to-detect useful signals and for making hypothesis decisions. All these parts of the signal processing are of course solved as numerical operations in the signal processor.
  • a demodulation specially adapted to the marking signal is now carried out in the previously calculated spectrum, with as many characteristic features as possible being determined.
  • Demodulation takes place in the frequency domain.
  • the method used is called the frequency autocorrelation function: S (f + ⁇ ) is the upper sideband, S (fX) the lower sideband and S (f) is the carrier, S * is the conjugate complex value.
  • This numerical synchronization signal demodulation is shown in FIG. 8. One sees that from the carrier r ' m (component of the vector for S (f)), and from the upper and lower sideband (Components of the vector r 'for S (f + ⁇ ) or S (fx)) is assumed.
  • the values and r ' m are in a frequency base value store 24.
  • the conjugate complex value of and of r ' m becomes r' m and multiplies and the results of these multiplications are added and subtracted, resulting in the values for the vector (numerical version of the frequency autocorrelation function) and for the error vector be formed.
  • These values are stored in corresponding memories 25 and 26 for the numerical version of the frequency autocorrelation function and for the error vector.
  • the noise stimulator is identified in FIG. 7a by reference numeral 20.
  • the decision about the hypotheses as to whether or not a synchronization signal is present must be assessed on the basis of the signal / noise ratio since the receiver knows neither signal energy based on noise power in advance.
  • the decision threshold is derived from the false alarm probability.
  • the determination of the noise (corresponds to the estimated value of the variance happens by means of the in the vicinity of r ' m , (Fig. 8) lie spectral support values and in this way provides a local power density in the vicinity of the synchronization signal.
  • the selected base values are demodulated in exactly the same way as the sidebands are demodulated in the demodulation described with reference to FIG. 8. Only is no longer equal to 16.
  • the noise stimulation is intended to be a combined variable x from noise energy and noise assessment in order to detect both "white" noise and interference signals in their influence.
  • the detected are normalized to the local noise variable (x) for each possible frequency and these normalized values (, ⁇ ) are sent to a decision maker 21, the following applying to the components of 1 and ⁇ :
  • a decision maker 21 the following applying to the components of 1 and ⁇ :
  • an accumulation in the form of digital filters is provided which uses the values 1 and ⁇ over several observation intervals, which leads to an improvement in the signal / noise ratio.
  • An example of such a filtering is in Fig. 7a designated by the reference numeral 22.
  • the gain can easily be 14 dB with an accumulation of 20 observation intervals.
  • False signals are signals similar to the synchronization signal with, for example, almost the same modulation frequency or a short presence time.
  • the demodulation carrier and the sidebands r ' m , or (Modulation frequency equal to 16 Hz) directly adjacent values a second noise statistic is formed and the two noise statistics are divided, the quotient deciding which noise statistic is to be used. In general, however, the combined variable x already mentioned is generated.
  • test variables Im, Al m , Im and ⁇ l ' m which result from N samples of a time function of duration T or more T, are checked in decision maker 21 (decision gate).
  • decision maker 21 decision gate 21
  • the quantities Im and Al m are first used for each frequency m (266 ⁇ m ⁇ 734).
  • the interval overlap is deliberately used in the fast Fourier transformation FFT in order to recover energy losses through the window function F.
  • the size a of the first test is itself a function of the noise statistics; For a certain number of noise support values used, an optimal threshold can be specified, which is stored in a RAM table. If Im or ⁇ lm does not meet the tests, these vector components are set to 0. The values l ' m and ⁇ l ' m are decided using the same method.
  • a so-called diversity combining (FIG. 7b) is necessary for the selected synchronization signal when using some type of diversity. It is important that there are known, rigid relationships between the individual signals during the synchronization signal processing. With the selected diversity pair consisting of 2 AM signals, only the phase shift of the modulation signals of 16 Hz by the angle 7T has to be taken into account thanks to the detector symmetry , ie one forms: and
  • a diversity combining 23 therefore only takes place if the hypothesis H 1 has been decided on both channels A and B. In the case of the combination, this results in a gain of 3 dB for the phase and frequency determination.
  • the use of frequency diversity is itself very profitable, since often a part of the channel is severely disturbed or suffers from fading.
  • the frequency and phase stimulation are summed up carried out. If H 1 is met at several locations on the frequency axis, then the frequency with the largest
  • the synchronization signal receiver operating according to the described method has the advantage that, thanks to the complete software real-time implementation of the receiver, many parameters can be optimized and varied; for example, the detection sensitivity can be optimized for a given estimation reliability.
  • the main advantages of the receiver are the great flexibility in the specification, in the age-free implementation and in achieving detection reliability that is close to that which is theoretically attainable. This is made possible by the operational sequence shown in FIG. 7 and by the digital signal processing, which only enables the required precision.
  • the system can be expanded to several transmission channels for scan operation without additional effort and micro-scan operation (division of a channel of 3 kHz width into 500 Hz subchannels) is also possible.
  • frequency and phase drifts can be continuously corrected after detection of the degrees of freedom, and instead of the synchronization signal a slow data connection can occur in a similar manner by replacing the now known degrees of freedom with new ones.
  • a selective call system can be created and a data modem for low baud rates can be derived from it, in which data occurs instead of the selective call address.
  • a connection is established without changing the channel, i.e. almost always guaranteed without any intervention from the synthesizer.
  • ECCM operation allows you to hide your own signal behind strong (e.g. hostile) transmitters. This prevents a rapid bearing or malfunction during network construction or during network control / network operation.
  • strong e.g. hostile

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Mobile Radio Communication Systems (AREA)
  • Radio Transmission System (AREA)
  • Radar Systems Or Details Thereof (AREA)
  • Radio Relay Systems (AREA)
  • Noise Elimination (AREA)
  • Communication Control (AREA)
  • Transceivers (AREA)
  • Input Circuits Of Receivers And Coupling Of Receivers And Audio Equipment (AREA)
  • Saccharide Compounds (AREA)
  • Steroid Compounds (AREA)
  • Near-Field Transmission Systems (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)
EP87105972A 1986-04-30 1987-04-23 Verfahren und Vorrichtung zum Verbindungsaufbau bei Kurzwellenfunknetzen Expired - Lifetime EP0243885B1 (de)

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AT87105972T ATE99101T1 (de) 1986-04-30 1987-04-23 Verfahren und vorrichtung zum verbindungsaufbau bei kurzwellenfunknetzen.

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CH1773/86A CH671124A5 (US20020098797A1-20020725-M00005.png) 1986-04-30 1986-04-30
CH1773/86 1986-04-30

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EP0243885A2 EP0243885A2 (de) 1987-11-04
EP0243885A3 EP0243885A3 (en) 1989-09-06
EP0243885B1 true EP0243885B1 (de) 1993-12-22

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EP (1) EP0243885B1 (US20020098797A1-20020725-M00005.png)
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CN (1) CN1009790B (US20020098797A1-20020725-M00005.png)
AT (1) ATE99101T1 (US20020098797A1-20020725-M00005.png)
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FI871843A (fi) 1987-10-31
EP0243885A3 (en) 1989-09-06
NO173760C (no) 1994-01-26
ATE99101T1 (de) 1994-01-15
NO173760B (no) 1993-10-18
NO871781L (no) 1987-11-02
US4853686A (en) 1989-08-01
CH671124A5 (US20020098797A1-20020725-M00005.png) 1989-07-31
CN1009790B (zh) 1990-09-26
NO871781D0 (no) 1987-04-29
FI86015B (fi) 1992-03-13
FI871843A0 (fi) 1987-04-28
DE3788531D1 (de) 1994-02-03
IL82068A0 (en) 1987-10-20
DK167418B1 (da) 1993-10-25
AU7226787A (en) 1987-11-05
JPS62262538A (ja) 1987-11-14
CN87103288A (zh) 1987-11-11
FI86015C (fi) 1992-06-25
AU596408B2 (en) 1990-05-03
EP0243885A2 (de) 1987-11-04
DK217687A (da) 1987-10-31
CA1269715A (en) 1990-05-29
DK217687D0 (da) 1987-04-29

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